This invention relates to an image processing apparatus using an interpolation method of 3-dimensional color space and an image forming apparatus for forming a color image by forming images of respective color components on a plurality of photosensitive drums by use of the above image processing apparatus and superposing the color images on recording paper.
Various media for dealing with color images, for example, a color facsimile and an image forming apparatus such as a color copying machine, color printer, optical disk, video recorder/player and television receiver are now developed. In the medium dealing with the color image, ideal color reproduction may be achieved by transmitting original image colors as they are as a final output. However, in a device dealing with color images, color signal spaces used for reproducing the original image colors are different in the input and output sections in many cases and it is absolutely necessary to convert the color signal space.
Conversion of the color signal space is represented as a general form by use of relational expressions indicating the relations between three color outputs and three color inputs. The color signal space converting process becomes extremely complicated because each color of the three outputs is based on the functional operation of the three input variables. Therefore, a look-up table (which is hereinafter referred to as LUT) method is used as a means for flexibly converting the color signal space for any type of function.
In the LUT method, all of the input-output relations are stored in a memory in the form of dictionary and a desired converted output can be immediately obtained by referring to the memory by use of a 3-color input signal. However, in the LUT method, a memory of a capacity as large as approx. 50 Mbytes in the case of normal full-color input of 8 bits.times.3 colors is required. If the memory of such a large capacity is used, a problem of high cost occurs, it takes a long time to form the table and it becomes impossible to quickly convert the color signal space for various purposes. Therefore, the recent color signal space conversion process utilizes an LUT interpolation method using an LUT memory of small capacity and a 3-dimensional interpolation operation unit.
Conversion output values on rough lattice points of the input color signal space are stored in the small-capacity LUT memory and precise output values for the rough conversion output values are derived by the interpolation operation by use of the 3-dimensional interpolation operation unit. By the above processes, the color signal space converting function which is approximately equivalent to that obtained when a large-capacity LUT memory is used can be attained.
As the 3-dimensional color space interpolation method, (a) a cube interpolation method, (b) hexahedron interpolation method, (c) pentahedron interpolation method, and (d) tetrahedron interpolation method are proposed. The cube interpolation method (a) is the simplest interpolation method and is attained by simply diving an input color space into unit cubes each including eight lattice points along the 3-color axes and effecting the interpolation process by using color conversion values on the lattice points to derive output values. On the other hand, the hexahedron interpolation method (b), pentahedron interpolation method (c), and tetrahedron interpolation method (d) are methods for further dividing the unit cube used in the cube interpolation method into hexahedrons, pentahedrons and tetrahedrons, respectively, and then effecting the interpolation process.
However, in the conventional 3-dimensional color space interpolation method, since the cube interpolation method uses the largest number (eight points) of lattice-point data items for the interpolation operation, the number of 3-dimensinal multiplication operations is large and the circuit scale will become large. Further, in the hexahedron interpolation method, pentahedron interpolation method and tetrahedron interpolation method, since the unit cube is divided into hexahedrons, pentahedrons and tetrahedrons, respectively, the number of lattice-point data items used for the interpolation operation is small and the precision of interpolation will become low. Further, since data items to be referred to on both sides of the boundary surface becomes different from each other as the result of division, the continuity of the interpolation data near or on both sides of the boundary surface will be lost.
As described above, in the image processing apparatus using the conventional 3-dimensional color space interpolation method, since the cube interpolation method uses the largest number (eight points) of lattice-point data items for the interpolation operation, the number of 3-dimensinal multiplication operations is large and the circuit scale will become large. Further, in the hexahedron interpolation method, pentahedron interpolation method and tetrahedron interpolation method, since the unit cube is divided into polyhedrons such as hexahedrons, pentahedrons and tetrahedrons, respectively, the number of lattice-point data items used for the interpolation operation is small and the precision of interpolation is low. Further, since data items to be referred to on both sides of the boundary surface of the polyhedron becomes different from each other as the result of division (because different polyhedrons are referred to), the continuity of the output result near the boundary surface is lost.